Решения:

17) $$1\frac{11}{70}+2\frac{13}{90} = 1\frac{99}{630}+2\frac{91}{630} = 3\frac{190}{630} = 3\frac{19}{63}$$

18) $$6\frac{3}{19}-4\frac{11}{38} = 6\frac{6}{38}-4\frac{11}{38} = 5\frac{44}{38}-4\frac{11}{38} = 1\frac{33}{38}$$

19) $$\frac{47}{64}+\frac{15}{64} = \frac{62}{64} = \frac{31}{32}$$

20) $$15\frac{13}{38}-9\frac{16}{57} = 15\frac{39}{114}-9\frac{32}{114} = 6\frac{7}{114}$$

21) $$3\frac{7}{10}+\frac{1}{10} = 3\frac{8}{10} = 3\frac{4}{5}$$

22) $$1-\frac{7}{36} = \frac{36}{36}-\frac{7}{36} = \frac{29}{36}$$

23) $$21\frac{5}{12}-11\frac{7}{8} = 21\frac{10}{24}-11\frac{21}{24} = 20\frac{34}{24}-11\frac{21}{24} = 9\frac{13}{24}$$

24) $$9\frac{9}{44}+1\frac{3}{44} = 10\frac{12}{44} = 10\frac{3}{11}$$

25) $$3-\frac{11}{12} = \frac{36}{12}-\frac{11}{12} = \frac{25}{12} = 2\frac{1}{12}$$

26) $$7-3\frac{3}{5} = 6\frac{5}{5}-3\frac{3}{5} = 3\frac{2}{5}$$